1. Field of the Invention
The present invention relates a communication apparatus, communication system, and communication control program which perform media access control (MAC) and, more particularly, to frame aggregation in which a plurality of media access control frame (MAC frame) are included in one physical frame.
2. Description of the Related Art
Media access control (MAC) is control for causing a plurality of communication apparatuses which perform communication while sharing the same medium to decide how to use the medium in transmitting communication data or management frame. Owing to media access control, even if two or more communication apparatuses transmit communication data (or management frame) by using the same medium at the same time, there is less chance of the occurrence of a phenomenon (collision) in which a communication apparatus on the receiving side cannot decode communication data. The fundamental access method of the IEEE802.11 MAC is CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). The CSMA/CA is designed to reduce the collision probability. Media access control is also a technique for controlling access from communication apparatuses to a medium so as to minimize the chance of the occurrence of a phenomenon in which, despite the presence of communication apparatuses having transmission requests, the medium is not used by any of the communication apparatuses.
However, especially in wireless communication, it is difficult to simultaneously monitor transmission data while the communication apparatus transmits the data, and therefore the media access control (MAC) is required in which collision detection is not assumed. A typical technique standard of wireless LAN IEEE802.11 adopts carrier sense multiple access with collision avoidance (CSMA/CA).
The MAC header has the duration value which is the time, in microseconds, required to transmit the data or management frame (including the time of SIFS interval). A communication apparatus which does not have any transmission rights regardless of the sequence judges a virtual busy state of the medium to thereby wait for the transmission. Therefore, the occurrence of the collision is avoided. In the IEEE802.11, it is defined that the state of the medium is judged by a combination of a virtual carrier sense of a MAC layer and a physical carrier sense of a physical layer to control the media access. The CSMA/CA is designed to reduce the collision probability.
In the IEEE802.11 in which the CSMA/CA is adopted, a communication speed has been increased by changing mainly a physical layer protocol. As to a 2.4 GHz band, IEEE802.11 (2 Mbps in 1997) has changed to IEEE802.11b (11 Mbps in 1999), and to IEEE802.11g (54 Mbps in 2003). As to a 5 GHz band, only IEEE802.11a (54 Mbps in 1999) has existed as a standard at present. Moreover, IEEE802.11 Task Group n (TGn) has been already set in order to establish a standard aiming at further speeding-up both in 2.4 GHz and 5 GHz bands.
In addition, several access control techniques designed to improve Quality of Service (QoS) are also known. For example, there is available HCCA (HCF Controlled Channel Access) which is an extended technique of a conventional polling sequence and is used as a QoS technique of guaranteeing parameters such as a designated bandwidth and delay time. According to the HCCA, in order to guarantee parameters such as bandwidth and delay time, scheduling is performed considering a required quality in the polling procedure. Jpn. Pat. Appln. KOKAI Publication No. 2002-314546 discloses a method of assigning priorities to communications between the communication apparatuses in a wireless network, while referring to QoS in the IEEE 802.11e standard.
Even when the communication speed of the physical layer is increased, there is a problem that substantial throughput of communication cannot be enhanced. That is, when the speeding-up of the physical layer is realized, a format of PHY (physical) frame is not efficient any more, and accordingly caused overhead obstructs the enhancement of the throughput. In the PHY frame, a time parameter concerning the CSMA/CA accompanies the MAC frame in a fixed manner. Moreover, a PHY frame header and PHY preamble are required for each MAC frame.
As a method of solving the problem of overhead and increasing throughput, a block response (Block acknowledgement) mechanism introduced in recently drafted IEEE 802.11e/draft 5.0 (enhancement of QoS in IEEE 802.11) is available. The block response mechanism can consecutively transmit a plurality of MAC frames without any random backoff (with SIFS interval), and hence can reduce the backoff amount to some degree. However, the overhead of a physical layer header and preamble cannot be effectively reduced. In addition, according to the aggregation technique introduced in initially drafted IEEE 802.11e, both the backoff amount and the physical layer overhead can be reduced. However, since the length of a physical layer frame containing MAC frames cannot be increased beyond about 4 kbytes under the conventional limitation on the physical layer, an improvement in efficiency is greatly limited. Even if the length of a PHY layer frame can be increased, another problem arises, i.e., a reduction in error tolerance.
Therefore, it has been necessary to solve the overhead accompanying the transmission of a plurality of frames is solved by an efficient frame format, and enhance the substantial throughput of the communication.
On the other hand, according to the conventional HCCA, quality can be guaranteed for each traffic stream, and data transmission corresponding to priority can be realized. The QoS is preferably utilized in a new communication system in which the throughput has been further enhanced. For example, the QoS is preferably used for frame aggregation designed to improve the transmission efficiency by transmitting a plurality of MAC frames upon containing them in one physical (PHY) frame. If, however, a conventional frame aggregation technique is simply applied to QoS like HCCA, the following problems arise.
That is, in the conventional frame aggregation technique in which no consideration is given to the priorities of frames, when a series of frames in a transmission queue (TxQ) are aggregation target frames, an FTP (File Transfer Protocol) frame with a relatively low priority may be extracted prior to a VoIP (Voice over IP) frame with a high priority and aggregated to a transmission aggregation frame. This may hinder the assurance of QoS in consideration of the priorities of frames.
Moreover, with regard to the procedure of a Partial Ack frame to designate some frames that have caused reception errors and to request retransmission, there is a problem that combined use with the ACK procedure (e.g., No acknowledgement (No Ack) procedure) inherent in QoS should be achieved.